Capacitive sense arrays may be used to replace mechanical buttons, knobs and other similar mechanical user interface controls. The use of a capacitive sense element allows for the elimination of complicated mechanical switches and buttons, providing reliable operation under harsh conditions. In addition, capacitive sense elements are widely used in modern customer applications, providing new user interface options in existing products. Capacitive sense elements can be arranged in the form of a capacitive sense array for a touch-sensing surface. When a conductive object, such as a finger, comes in contact or close proximity with the touch-sensing surface, the capacitance of one or more capacitive touch sense elements changes. The capacitance changes of the capacitive touch sense elements can be measured by an electrical circuit. The electrical circuit converts the measured capacitances of the capacitive sense elements into digital values.
Transparent touch screens that utilize capacitive sense arrays are ubiquitous in today's industrial and consumer markets. They can be found on cellular phones, GPS devices, cameras, computer screens, MP3 players, digital tablets, and the like. In contemporary cellular phones and smart phones, touch screen area is of significant concern to manufacturers given the small amount of space available for user interaction. As such, manufacturers seek a touch screen that maximizes its useable area while maintaining uniform position tracking accuracy. However, conventional designs exhibit considerable position tracking error near the edges of the touch screen.
FIG. 1 illustrates a conventional pattern design of a capacitance sense array panel 100 having homogenous width stripes or sense elements. The capacitance sense array panel 100 comprising an N×M sense element matrix which includes transmission (“Tx”) sense element 102 and receive (“Rx”) sense elements 104. The transmission and receive sense elements in the N×M sense element matrix are arranged so that each of the transmission sense elements intersects each of the receive sense elements. Thus, each transmission sense element is capacitively coupled with each of the receive sense elements. For example, transmission sense element 102 is capacitively coupled with receive sense element 104 at the point where transmission sense element 102 and receive sense element 104 intersect. The intersection of the transmission sense element 102 and the receive sense element 104 is called a sense element. It is noted that the embodiment disclosed in FIG. 1, the orientation of the axes of Tx sense elements may be switched with the Rx sense elements as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure.
Because of the capacitive coupling between the transmit and receive sense elements, a Tx signal (not shown) applied to each transmit sense element induces a current at each of the receive sense elements. For instance, when a Tx signal is applied to transmission sense element 102, the Tx signal induces an Rx signal (not shown) on the receive sense element 104. When a conductive object, such as a finger, approaches the N×M sense element matrix, the object will modulate the signal by changing the mutual capacitance at the junction, or the intersect point between the Tx and Rx sense elements. Since a finger would normally activate about three to five neighboring junctions, a signal profile can be readily obtained. Finger location can therefore be determined by the distribution of this profile using a centroid algorithm.
As shown in FIG. 1, the Tx sense elements 102 and Rx sense elements 104 have sensor widths which are uniformly across the panel. These are referred to as homogenous width sense elements. One problem with this conventional design is that accuracy variation between a center area and an edge area. The edge area is often defined as within one sensor pitch from the physical edge of the touch panel, and elsewhere is called center area. The dimension of sensor pitch is typically the width of one sense element. In touch panel applications, accuracy is defined as error between the location of a conductive object on or in proximity to the touch panel and the location sensed by the touch panel. Conventionally, accuracy within the center area is significantly greater than the accuracy within the edge area. For example, the accuracy of this pattern along the edges of the conventional capacitive sense array is often at least three times worse than the accuracy in the central area of the array. The major reason for this significant difference in accuracy is that with the finger landing on an edge sensor causes a signal profile to be chopped off at the edge of the panel. So, without a complete signal profile, the centroid determination of the finger would certainly have some error in the centroid algorithm since the information is unbalanced, resulting in systematic centroid offset along the edges of the panel.